The invention relates to performance characteristics stabilization for a radial compressor.
The trend in the development of charged engines is today towards higher medium pressures even at low engine speeds. When using present-day conventional compressors the engine operating range comes very close to the surge line and moves in the noise margin partially preceding the surge line.
To improve the capability of controlling such motors, compressors ape necessary having a characteristic exhibiting a wide performance range and wide range of high efficiency.
To meet the aforementioned requirements with existing hardware, it is possible to use range-stabilizing (RS) steps in the inlet passage of the compressor which would be very effective.
Such performance characteristic stabilizing features in the form of venting chambers have been known for a long time. They are effective in operating ranges in which the flow angle of attack on the impeller wheel is not correct. The performance characteristic stabilization permits in such critical operating points a stabilization of the performance characteristic by compensating for such disturbances by the buffer volume in the venting space, If the disturbance is more pronounced, a circulation occurs between the annular slots and the preventing space. In the region of the surge line the impeller wheel is subjected to flow with increasingly smaller angle of attack and in addition the pressure in the impeller wheel rises. As a result, air mass is conveyed back to the compressor inlet. At the impeller inlet edge more air is sucked in than the compressor as a whole conveys. As a result the angle of attack for this operating point is improved and the surge line shifted to smaller flow rates. The choke margin is caused by reaching the velocity of sound at the impeller inlet edge. There, a lower pressure is generated so that, via the bypass conduit, air is conveyed into the impeller wheel, whereby the choke margin is shifted to the right. In between, the performance characteristics stabilization arrangement is more or less ineffective. With ideal attack and matching it is fully ineffective.
The technique of compensating the pressure by bypass conduits which are connected to various axial regions and via which a pressure equalization can take place is known in particular from DE-PS 1,428,077. The technique has been progressively further developed as explained in a summary article by H. D. Henssler (Kuhnle, Kopp & Kausch, special print in VGB Kraftwerkstechnik, 57th edition, no. 3,1977).
Modern means for performance characteristics stabilization are known from EP-A 348674, EP-B 229519 and GB-OS 2,220,447. EP-B 229519 and GB-OS 2,220,447 disclose a bypass conduit leading directly from the gas intake to behind the leading edge of the impeller. The through the venting space is determined by the pressure difference in front of the impeller leading edge via an opening to the venting space, which hereinafter is referred to as opening 1, or from the venting space to the pressure in an opening at the impeller wheel, referred to hereinafter as opening 2.
A disadvantage is that the conditions in the venting space do not correspond to the conditions in the gas intake directly in Front of the impeller leading edge. For adjustment only the groove can be used as essential control point. Thus, a wide groove could appreciably shift the choke margin but in the region of the optimum this would considerably impair the efficiency and consequently the limit of such a design would be reached with the tolerability of the loss in efficiency.
These negative properties are avoided in EP-A 348674 in that both the inlet and the outlet lie almost perpendicular to the main Flow. The bypass conduit is thus not directly attacked. This results in a bypass flow which is generated by the pressure differences at the inlet and outlet of the bypass conduit.
The disadvantage of this construction is due to the fact that both sides of the bypass conduit lie in front of the impeller wheel. This means that the pressure difference at the bypass conduit is in any case very small and consequently this design is effective only when extreme pressure gradients occur in front of the impeller wheel. It is however desirable For the stabilization to start much earlier because the characteristic is then broadened in the range of high delivered volumes as well. For the normal operating point of an engine this means a better efficiency at lower speed level or greater reserves in the higher speed range.
A Further disadvantage of known designs resides in that the stabilizing means must be adapted to the type of compressor. Differences in the compressor blade design, contour variations and resulting different positions and intensities of disturbance or surge field at did not hitherto make it possible to give clear technical guidelines for designing a stabilizing means. Nor was hitherto possible to predict reliably whether a stable range could be achieved at a 11, and which stabilizing measures, in given compressor, in particular in a radial compressor, would be effective. With the present state of the art it would be extremely desirable if adaptation could be achieved by varying a minimum number of parameters.
These disadvantages lead to the object of the invention, that is, to provide a performance characteristics stabilization for radial compressors which permits a widening of the range without losses of efficiency.